At the ard fheis in Castlebar at the weekend, Martin McGuinness turned what had been a vague threat into a definite commitment.

"Let me be clear; Sinn Fein will resist this onslaught on the most vulnerable," said the Deputy First Minister.

"We will not tolerate the introduction of a 'bedroom tax'. We will deploy a petition of concern on this clause if it is brought to the floor of the Assembly."

The SDLP welcomed his statement, and revealed that it already handed in its own petition of concern last week.

The petition means that the measure must secure a majority of both nationalists and unionists voting separately to pass into law at Stormont. Sinn Fein needs the support of just one SDLP MLA to give it the 30 Members it needs to force the issue.

Theresa Villiers, the Secretary of State, told the Belfast Telegraph the Government is unlikely concede on this issue.

"I know that the bedroom subsidy is controversial in Northern Ireland but the reality is we face difficult choices on welfare" she said. "I think that is unlikely (to be changed) because it is a pretty fundamental part of the reform," she said last week.

The bedroom tax measure penalises benefit recipients in rented accommodation with spare bedrooms to encourage them to move to smaller homes. However, there are not enough smaller dwellings here to meet demand.

The DUP warned against breaking parity with the benefits system in Britain, which could leave the Executive with a financial black hole to fill by making cuts in other services.

There could also be the expense of staff and software to administer a different system.